1. Field of the Invention
The present invention relates to a current-source sine wave voltage inverter and a fuel cell system using the same, more particularly, to a current-source sine wave voltage inverter using voltage-clamping and soft-switching techniques, which converts a direct current (DC) source to an alternating current (AC) sine wave voltage using a difference between AC sine wave voltage command and feedback voltage to control the turn-on time of switches, and using an inductor to generate a current source to charge capacitors via positive/negative cycles of full-bridge switch to adjust a rise and fall range of voltage to provide linear voltage regulation.
2. Description of the Prior Art
Presently there are two kinds of products which can convert a DC source to a 60 Hz AC voltage. One is an inverter of AC motor, which uses a coil inductor of motor and a PWM (pulse width modulation) technique to generate a sine-wave-like current. However, it is not suitable for resistive or capacitive loads, therefore basically, the inverter is not for home appliances or computer products. The other is a modification of the inverter, such as UPS (Un-interruptible Power Supply), which adds a LC filter circuits at the output and a feedback circuit to provide a constant voltage, regardless of various loads and input voltages. Besides, batteries and charging/discharging circuits are often included to provide a back-up power source in addition to a utility power. Nowadays Taiwan has become a leader in UPS products and related techniques, nevertheless, there are a few issues remained to be solved. Firstly, an output current will have to pass through a filter inductor, and in consideration of the −3 dB response of second order resonance circuit. A common UPS has mH-level of inductance, therefore the filter inductor will increase the product's weight and energy transfer loss. Secondly, the voltage L·di/dt between two ends of the filter inductor is the difference between a DC voltage and an output voltage. While the minimum value occurs around a sine wave peak, the output waveform tends to distort around the sine wave peak turning point and generate high-frequency harmonic components due to the filter inductor. Therefore, it is inevitable even with a higher filter voltage. Though the inductor is intended for a filter use, it also limits the regulation ability under loads being varied suddenly. Thirdly, a few kinds of loads, such as half-wave rectifying loads or highly inductive loads, could harm the driving circuit due to the symmetry of an LC filter circuit waveform, and highly inductive loads could change the frequency response of a second order filter circuit as well. A DC voltage level have to be raised in case the output sine wave voltage is too low, and consequently the system could be damaged due to an overly high voltage. Fourthly, the voltage distortion rate of non-resistive loads, generally refers to Total Harmonic Distortion (THD), is far more higher than resistive loads, because the traditional second order filter circuit is not capable of handling non-resistive loads, such as inductive, capacitive and nonlinear loads.
Besides, switching loss increases as switching frequency rises, which in turn decreases the system efficiency, many manufacturers started to apply various soft-switching techniques to high power IGBT switches, several papers prove that decreases the PWM switching loss will help to increase switching frequency and improve output voltage waveform.
Compared to traditional PWM methods, the sine wave voltage of current source inverter is mostly used for charging the capacitor to accumulate sine wave voltage under various loads and frequency changes. However, due to the large inductor used in current source, it's hard to control the inductor circuit and to realize soft-switching techniques, resonant voltage and high current issues are difficult to overcome. Recently the Institute of Electrical and Electronic Engineers (IEEE) proposed a voltage-clamping technique to handle current source inverter, the circuit exhibited soft-switching characteristics, which also restrains the switch voltage under the factor of 4, nevertheless, the imaginary part of the inductor current in the current source is too high, it's hard to decrease the volume. Besides, issues such as high ripples in the voltage waveform, no field experiment available and the driving object being an inductance motor still exist.
Therefore, the above-mentioned devices present several shortcomings to be overcome.
In view of the above-described deficiency of prior-art devices, after years of constant effort in research, the inventor of this invention has consequently developed and proposed a current-source sine wave voltage inverter using voltage-clamping and soft-switching techniques.